1. Field of the Invention
The invention generally relates to a semiconductor laser device and a manufacturing method thereof. More particularly, the invention relates to a monolithic semiconductor laser device for emitting laser light of a plurality of wavelengths and a manufacturing method thereof.
2. Background Art
A semiconductor laser device is used as a pickup light source of an optical disc device and a light source for optical information processing, optical communication and optical measurement. For example, an infrared laser device having a 780 nm (nanometer) wavelength band is used as a pickup light source for playing back and recording CDs (Compact Discs) and MDs (Mini Discs). A red laser device having a 650 nm wavelength band is used as a pickup light source for playing back and recording higher density DVDs (Digital Versatile Discs).
There has been a demand for a drive that is compatible with a plurality of types of optical discs such as CDs, MDs, and DVDs. In this case, an infrared laser device and a red laser device need to be mounted on a single drive. However, with recent demand for reduced size and cost, simplified optical adjustment and assembly process, and the like of a drive, an optical integrated unit including a pickup light source needs to be further simplified. Therefore, a dual-wavelength semiconductor laser device in which an infrared laser element having a 780 nm wavelength band and a red laser element having a 650 nm wavelength band are integrated on a common substrate has been used in practical applications (e.g., Japanese Laid-Open Patent Publication No. 2001-57462), which has contributed to significant simplification of an optical integrated unit.
In the future optical disc market, optical discs are required to adapt to LS (LightScribe) for drawing pictures and characters on media labels by using a CD infrared laser and to higher speed DVDs. Improvement in output power of a dual-wavelength laser is essential to meet such a demand.
In order to implement a higher power semiconductor laser device, optical damage at an end face, that is, COD (Catastrophic Optical Damage) degradation, needs to be suppressed. The use of an end face window structure is effective to suppress COD degradation. In formation of the end face window structure, impurities are diffused in an active layer near the light emitting end face of a laser element for equalization of the composition, whereby an effective bandgap near the end face is widened. Light absorption near the end face is thus suppressed by the end face window structure. Accordingly, a negative chain reaction of heat generation near the end face due to light absorption, reduction in bandgap due to the heat generation, and further light absorption due to the reduction in bandgap can be prevented, whereby COD degradation is suppressed.
The structure of a dual-wavelength semiconductor laser device is roughly divided into two types: a hybrid dual-wavelength semiconductor laser device; and a monolithic dual-wavelength semiconductor laser device. The hybrid dual-wavelength semiconductor laser device is fabricated by first producing an infrared laser element and a red laser element individually and then integrated both laser elements on a common substrate in a packaging process. The monolithic dual-wavelength semiconductor laser device is fabricated by producing an infrared laser element and a red laser element on a common substrate. In view of the requirement for higher accuracy of the distance between respective light-emitting points of the infrared laser element and the red laser element, and the mounting yield, the monolithic dual-wavelength semiconductor laser device has become mainstream.
However, the monolithic dual-wavelength semiconductor laser device has a problem that it is difficult to maximize characteristics of each laser element.
In order to implement a higher power monolithic dual-wavelength semiconductor laser device, stable operation at high temperature and high power needs to be assured in addition to the above-mentioned suppression of the COD degradation. Reduction in operating current is important to obtain stable operation at high temperature and high power. Suppression of carrier overflow in the active layer is necessary to reduce the operating current.
In an infrared laser element having an active layer made of an AlGaAs (aluminum gallium arsenide)-based material, carrier overflow can be suppressed by using a cladding layer made of an AlGaInP (aluminum gallium indium phosphide)-based material having a wide bandgap. However, if impurity diffusion is excessively conducted in the AlGaInP-based cladding layer in the process of forming an end face window structure, a wavelength shift amount (the difference in wavelength between a gain region and a window region) is reduced. Excessive impurity diffusion causes pileup of impurities in the active layer. Such impurity pileup causes an impurity state in the active layer and contamination of the active layer with indium (In) from the cladding layer, thereby reducing the bandgap in the end face window region. As a result, light absorption in the end face window structure is increased and characteristics of the infrared laser element such as COD are degraded.
On the other hand, in a red laser element having an active layer made of an AlGaInP-based material, problems such as reduction in wavelength shift amount as in the infrared laser element does not occur when the cladding layer is made of AlGaInP. Therefore, it is preferable to form a large end face window structure by diffusing impurities in the cladding layer as much as possible. However, if impurity diffusion reaches a GaAs (gallium arsenide) buffer layer beyond the AlGaInP-based cladding layer, a leakage current is generated and characteristics of the semiconductor laser are degraded.
In order to obtain a higher power monolithic dual-wavelength semiconductor laser device, the impurity diffusion process for forming the end face window structure needs to be optimized for each of the infrared laser element and the red laser element.
On the other hand, a simplified manufacturing process and improved yield are essential for cost reduction of the monolithic dual-wavelength laser device. The impurity diffusion process for forming the end face window structure involves heat history. Therefore, when the infrared laser element and the red laser element are individually formed with heat history in the monolithic dual-wavelength semiconductor laser device, excessive impurity diffusion may occur in the laser gain portion, which may result in reduction in reliability. Accordingly, it is desirable that the end face window structure can be stably formed in the infrared laser element and the red laser element simultaneously. Moreover, it is preferable that process conditions are standardized even when the window structure is formed individually in the infrared laser element and the red laser element.